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Multiple FM station receiver

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#1
Hi everyone,

This is my first time experimenting with SDR and GNURadio, so please bear with me here.

I started with this simple tutorial I found on RTL-SDR.com: Tutorial: Creating an FM Receiver in GNURADIO using an RTL-SDR source - rtl-sdr.com

So this basically shows how to tune your RTL-SDR to an FM station, demodulate it, and play the audio. Simple and fun to do, got it working pretty quickly.

So now I've read that the RTL-SDR has a bandwidth of ~2.4 GHz. So in theory, it should be possible to capture the full spectrum of 106.1MHz - 108.5MHz, and then concurrently split that captured spectrum into a bunch of 100KHz bands that correspond to FM stations, and demodulate them individually and play multiple stations at once, right?

So let's assume I tune my RTL-SDR to 106.1MHz. I want to capture the stations at 106.5 and 106.9MHz. I would think that I need to split the signal coming out and feed it through two bandpass filters:

1) low: 400KHz (106.5 - 106.1), high: 500KHz (106.6 - 106.1)

2) low: 800KHz (106.9 - 106.1), high: 900KHz (107 - 106.1)

And then pipe each of those through a WBFM receiver, then to audio output. I have tried this as shown in this flowgraph:


But it's just giving me a bunch of static. I have a feeling that this has something to do with the WBFM Receive module expecting the FM signal to be in the first 100KHz of its input, but that part has nothing because of the bandpass filter. Do I need to "shift" this input signal somehow? Any advice on the correct way to make this work would be greatly appreciated!

Also, what is a reasonable value for "Transition Width" in a bandpass filter for this?
 
Joined
Sep 30, 2014
Messages
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#2
AHA! I have figured it out. I needed to use the "Frequency Xlating FIR Filter" block to center each station. My resulting radio design looks like this:


If there's anything stupid I'm doing in this design, please let me know!
 
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#3
Hi everyone,
So now I've read that the RTL-SDR has a bandwidth of ~2.4 GHz.
That should be MHz, not GHz. So you can (if you have the CPU power) receive and demodulate multiple broadcast FM stations simultaneously, you can't cover the entire FM broadcast frequency range; only stations that fall within a ~2MHz window. You can choose the location of that window.
 
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